Unless I'm mistaken, your base assumption is that scripture teaches that vows, no matter what their content, are never, ever to be broken.

I don't think the case for that is anywhere near clear enough to create a problem with "shall we engage in abominable conduct or break this vow?"

What I can think of to support the position are a few exemplary cases of vows not being broken in varying circumstances and never (as far as I can tell) an explicit word from God requiring such to be the case, one commendation of someone who keeps vows to his own hurt (which is not the same as keeping them to the dishonor of God's law) versus explicit commands not to engage in perversion, murder, what have you.

So I'm not sure how you get a clear enough teaching on not breaking vows to allow it to override situations of obvious law-breaking. Even among your examples, the closest thing you get is Jephthah, but Jephthah did a lot of stuff that doesn't really work for normative example.

~Jane~

Suddenly I was abuzz with ideas. What about issuing stickers printed with the words, "This apostrophe is not necessary"? What about telling people to shin up ladders at dead of night with an apostrophe-shaped stencil and a tin of paint? Why did the Apostrophe Protection Society not have a militant wing? Could I start one? Where do you get balaclavas? -- Eats, Shoots and Leaves